Composite aircraft manufacturing tooling and methods using articulating mandrels

ABSTRACT

Composite tooling systems and methods of manufacturing composite stringers of aircraft are provided herein. A composite tooling system includes a layup tool and an articulating stringer mandrel. The layup tool has a contoured surface with portions having different surface complexities. The articulating stringer mandrel includes two rigid mandrel elements and a cable. The first rigid mandrel element defines a first aperture, a first stringer surface, and a first bottom surface. The second rigid mandrel element is adjacent to the first rigid mandrel element and defines a second aperture, a second stringer surface, and a second bottom surface. A first length of the first rigid mandrel element cooperates with the surface complexity and a second length of the second rigid mandrel element cooperates with the surface complexity at the second rigid mandrel element so that a gap between the rigid mandrel elements does not exceed a predetermined gap threshold.

TECHNICAL FIELD

The present invention relates generally to tooling and methods formaking composite reinforced structures for aircraft, and moreparticularly relates to aircraft wing tooling and methods witharticulating stringer mandrels having rigid mandrel elements.

BACKGROUND

The fuselage, wings, and empennage of an aircraft typically includestringers that are coupled to skin that forms the smooth aerodynamicouter surfaces of the fuselage, wings, and empennage. The stringers andskin cooperate to provide flexural and torsional stiffness to thesesections of the aircraft. Traditionally, the fuselage, wings, andempennage surfaces and the associated stringers are fabricated frommetal, such as aluminum, steel, or titanium.

Fiber reinforced composite materials are widely used in a variety ofcommercial and military aircraft products as a substitute for metals,particularly in applications where relatively low weight and highmechanical strength are desired. The material is generally comprised ofa network of reinforcing fibers that are arranged in layers or plies.The layers include a resin matrix that substantially wets thereinforcing fibers and that is cured to form an intimate bond betweenthe resin and the reinforcing fibers. The composite material may beformed into a structural component by a variety of known formingmethods, such as extrusion, vacuum bagging, autoclaving, and/or thelike.

As the skins and stringers for various sections of aircraft transitionfrom metallic materials to fiber reinforced composite materials,multiple issues have arisen. In a current fabrication process known asPre-cure/Co-bond or Co-bond, a fiber reinforced composite skin is formedby stacking layers together that contain reinforcing fibers in a resinmatrix. Typically, some of the layers are staggered relative to eachother so that the stack conforms to a desired contoured or taperedgeometry. The stacked layers are heated and pressurized to cure thepolymeric resin matrix and form a precured skin. A stringer mandrel isplaced on the stacked layers and stringer plies are stacked onto thestringer mandrel. Pressure and heat are applied to cure the stringerpreform using the tooling to form a fiber reinforced composite stringerthat is attached to the precured skin. Unfortunately, defects oftenoccur along the interface between the precured skin and the fiberreinforced composite stringer. In particular, the skin typically has acontoured outer surface that includes small steps or drop-offs that areformed by the gaps between mandrel elements of the stringer mandrel.Large gaps cause under compressed areas, e.g., voids, and overcompressed areas, e.g., resin poor areas, within the composite stringer.These under and over compressed areas can reduce the load transferefficacy between the precured skin and the fiber reinforced compositestringer, thereby reducing the rigidity and support provided by thefiber reinforced composite stringer.

The stringer is then Co-cured over the pre-cured skin in a second curingcycle. Pre-curing provides good skin quality due to uniform baggingpressure and relatively simple geometry, but requires two runs of theautoclave process and preparation. Requiring two runs becomes veryexpensive and time consuming, and introduces risk associated withre-heating the skin laminate.

Another current fabrication method is Co-cure, which allows creating acomplex part in a single autoclave curing step. The disadvantage ofCo-cure is that bagging over complex shapes creates a non-uniformpressure where the laminate quality can suffer by having resinrich/starved areas, and potential ply distortion due to uneven pressure.

A third current fabrication method is Secondary Bonding, in which all ofthe constituent components are pre-cured, and then assembled in a bondmold with adhesive and cured together. Secondary Bonding provides thebest laminate quality for all parts, but requires multiple cure cycles,increased of preparation, extra tooling, and increased difficulty withfit-up issues of pre-cured parts.

Conventional mandrels include foam mandrels and elastomeric mandrels.Foam mandrels are difficult to remove from the fiber reinforcedcomposite stringer after curing and are often considered “flyaway”tooling that remains in the composite wing. Elastomeric mandrels areeasier to remove from the cured composite stringer, but do not offer thestiffness and rigidity required for well-defined edges in the compositestringer. Furthermore, the elastomeric mandrels are typically onlyavailable for several uses before the elastomeric material beginsbreaking down and is not able to withstand the curing process.

Accordingly, it is desirable to provide composite tooling systems andmethods of manufacturing composite stringers that are removable from thestringer after curing, are durable, and provide the rigidity to producewell-defined edges on the cured composite stringer. Furthermore, otherdesirable features and characteristics of the present invention willbecome apparent from the subsequent detailed description and theappended claims, taken in conjunction with the accompanying drawings andthis background.

BRIEF SUMMARY

Composite tooling systems and methods of manufacturing compositestringers of aircraft are provided herein. In accordance with anexemplary embodiment, a composite tooling system includes a layup tooland an articulating stringer mandrel. The layup tool has a contouredsurface with a first portion with a first surface complexity and asecond portion with a second surface complexity. The articulatingstringer mandrel includes a first rigid mandrel element, a second rigidmandrel element, and a cable. The first rigid mandrel element defines afirst aperture, a first stringer surface to which the composite stringerconforms, and a first bottom surface opposing the first portion of thecontoured surface. The first rigid mandrel element has a first lengthalong a longitudinal direction of the composite stringer. The secondrigid mandrel element is adjacent to the first rigid mandrel element anddefines a second aperture, a second stringer surface to which thecomposite stringer conforms, and a second bottom surface opposing thesecond portion of the contoured surface. The second rigid mandrelelement has a second length along the longitudinal direction. The cableextends through the first aperture and the second aperture. The firstlength cooperates with the first surface complexity and the secondlength cooperates with the second surface complexity so that a gapbetween the first rigid mandrel element and the second rigid mandrelelement does not exceed a predetermined gap threshold.

In accordance with another exemplary embodiment, a method ofmanufacturing a composite stringer of an aircraft includes laying skinplies on a layup tool that includes a contoured surface having a firstportion with a first surface complexity and a second portion with asecond surface complexity. The method further includes placing anarticulating stringer mandrel on the skin plies. Placing thearticulating stringer mandrel further includes positioning a first rigidmandrel element on the skin plies so that a first bottom surface of thefirst rigid mandrel opposes the first portion, where the first rigidmandrel defines a first aperture and a first stringer surface to whichthe composite stringer conforms. The first rigid mandrel element has afirst length along a longitudinal direction of the composite stringer.Placing the articulating stringer mandrel further includes positioning asecond rigid mandrel element on the skin plies adjacent to the firstrigid mandrel element so that a second bottom surface of the secondrigid mandrel element opposes the second portion, the second rigidmandrel defining a second aperture and a second stringer surface towhich the composite stringer conforms. The second rigid mandrel elementhas a second length along the longitudinal direction. Positioning thefirst rigid mandrel is based at least in part on the first length andthe first surface complexity and positioning the second rigid mandrel isbased at least in part on the second length and the second surfacecomplexity so that a gap between the first rigid mandrel element and thesecond rigid mandrel element does not exceed a predetermined gapthreshold. The method further includes tightening a cable that extendsthrough the first aperture and the second aperture. The method furtherincludes laying stringer plies overtop the articulating stringer mandreland curing the skin plies and the stringer plies.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be described in conjunction withthe following drawing figures, wherein like numerals denote likeelements, and wherein:

FIGS. 1A, 1B, 1C, and 1D are views of a composite tooling system inaccordance with an exemplary embodiment;

FIG. 2 is a sectional view of the composite tooling system depicted inFIG. 1 with exaggerated dimensions for ease of explanation;

FIG. 3 is a front view of a rigid mandrel element of the compositetooling system depicted in FIG. 2;

FIG. 4 is a front view of a mandrel ball of the composite tooling systemdepicted in FIG. 2; and

FIG. 5 is a flow diagram for a method manufacturing a composite stringerof an aircraft in accordance with an exemplary embodiment.

DETAILED DESCRIPTION

The following Detailed Description is merely exemplary in nature and isnot intended to limit the invention or the application and uses of theinvention. Furthermore, there is no intention to be bound by any theorypresented in the preceding background or the following detaileddescription.

Various embodiments contemplated herein relate to tooling formanufacturing fiber reinforced composite stringers, and to methods formaking such reinforced composite stringers. In the example provided, thefabrication process is a Co-cure process. Unlike the prior art, theexemplary embodiments taught herein arrange rigid mandrel elements of anarticulating mandrel based on a surface complexity of the wing at eachportion of the stringer, which addresses the conventional processshortcomings of mandrel distortion and uneven pressure, which causes plydistortion. Each rigid mandrel element has dimensions that are suitablefor disposition over a portion of the wing tooling with a surfacecomplexity that interacts with the dimensions of the rigid mandrelelement. The interaction causes misalignment of adjacent mandrelelements, resulting in a gap between edges of the adjacent rigid mandrelelements. The dimensions of the rigid mandrel elements are smaller atportions of the tooling that have higher surface complexity so that thegap does not exceed a gap threshold. By keeping the gap below the gapthreshold, stepping and other undesirable composite deficiencies may bereduced.

Referring to FIGS. 1A-D and 2-4, views of a composite tooling system 100and components of composite tooling system 100 are provided inaccordance with an exemplary embodiment. FIGS. 2-4 have exaggerateddimensions when compared with FIGS. 1A-D for ease of explanation of thefeatures of composite tooling system 100. Composite tooling system 100may be used to manufacture a composite stringer 102 and composite skin104 of an aircraft, as can be seen in FIG. 3. Composite stringer 102 andcomposite skin 104 are each formed of fiber reinforced compositematerial. As illustrated, in accordance with an exemplary embodiment,composite stringer 102 comprises a beam body portion 106 and two runoutportions 108 that extend from beam body portion 106 in oppositedirections. Composite stringer 102 generally follows the contours ofcomposite skin 104.

Composite tooling system 100 includes a layup tool 110 and anarticulating stringer mandrel 112. Layup tool 110 defines a contouredsurface 120 that has a first portion 122 and a second portion 124. Firstportion 122 has a first surface complexity and second portion 124 has asecond surface complexity that is less than the first surfacecomplexity. As will be appreciated by those with ordinary skill in theart, the surface complexity refers to the rate of change of the slope ofcontoured surface 120. Surface complexity also includes ramps due to plydrops under the mandrel. Accordingly, a portion with a high surfacecomplexity has a slope that changes more quickly than does a portionwith a low surface complexity. For example, a flat surface has nosurface complexity.

Articulating stringer mandrel 112 includes a first rigid mandrel element130, a second rigid mandrel element 132, a third rigid mandrel element134, a cable 136, and a plurality of mandrel balls 138. FIGS. 1A-Cillustrate additional end elements 137 that are provide a bearingsurface for tightening articulating stringer mandrel 112 with cable 136.The example provided includes a plurality of first rigid mandrelelements 130 and a plurality of second rigid mandrel elements 132. Thenumber of rigid mandrel elements 130, 132, 134 may vary based on theparticular stringer and the shape of the skin to be manufactured, aswill be appreciated by those with ordinary skill in the art.

The arrangement of rigid mandrel elements 130, 132, 134 is based on thesurface complexity of contoured surface 120 and on the dimensions (e.g.,length and/or width) of each rigid mandrel element 130, 132, 134 toensure that a gap 139 between each rigid mandrel element 130, 132, 134does not exceed a predetermined gap threshold. The predetermined gapthreshold is based on composite process characteristics that impactstepping in the composite stringer. For example, the gap threshold maybe based on reducing stepping based on resin properties, autoclavepressures, or other composite process characteristics. While the mandrelelements shown can be substantially the same, it is possible to reducestepping further by sanding or otherwise dressing the joints betweenrigid mandrel elements after the mandrel assembly is at its proper shapeand the cable is tightened.

Rigid mandrel elements 130, 132, 134 and mandrel balls 138 are formedfrom a rigid material. As used herein, the term “rigid material” refersto a material that substantially maintains shape during an autoclaveprocess to cure the composite material. In the example provided, therigid material is a metal material, such as aluminum or invar. In someembodiments, the rigid material has a Young's modulus that is greaterthan about 10 GPa. In some embodiments, the rigidity of materials couldbe metal with Young's moduli from 10×10^6 to 30×10^6 psi. In contrast,flexible materials—such as elastomeric materials—that readily deformunder autoclave temperatures and pressures are explicitly excluded fromthe preceding definition of “rigid material.” As will be appreciated bythose with ordinary skill in the art, the specific rigid material mayvary based on the temperatures and forces involved in any particularautoclave process. In some embodiments, rigid polymers with higherdurometers than flexible rubber may be used, such that if a mandrel weremade one piece, it would not be flexible enough to allow extraction fromthe cavity. In general, the material is rigid enough to maintain shape,and tough enough for re-use without damage during extraction.

The rigid material may also be at least partially selected based on theexpansion characteristics required. Invar or carbon composite mandrelshave low expansion relative to the composite part, but are then moredifficult to remove. In some embodiments, rigid mandrel elements 130,132, 134 and mandrel balls 138 have an elastomeric coating.

First rigid mandrel element 130 defines a first cavity 140, an aperture142, a first cross section perimeter 144, and a first bottom surface145. Each longitudinal end of first rigid mandrel element 130 includes afirst cavity 140 to receive a mandrel ball 138. In the example provided,first cavity 140 is defined by a spherical surface that opposes eachmandrel ball 138. The spherical surface accommodates rotation andlongitudinal misalignment of adjacent rigid mandrel elements 130, 132,134 so that articulating stringer mandrel 112 may conform to contouredsurface 120.

Aperture 142 extends through a longitudinal length of first rigidmandrel element 130 between first cavities 140 on each side of firstrigid mandrel element 130. Aperture 142 accommodates cable 136 tofacilitate assembly of articulating stringer mandrel 112.

First cross section perimeter 144 defines a shape of composite stringer102, which conforms to first cross section perimeter 144 duringautoclave curing. The shape of first cross section perimeter may varybased on the particular implementation and desired stringer shape. Inthe example provided, first cross section perimeter 144 is substantiallytrapezoidal in shape.

First bottom surface 145 opposes first portion 122 of contoured surface120. First rigid mandrel element 130 has a first length 146 along thelongitudinal direction of articulating stringer mandrel 112 andcomposite stringer 102. First length 146 is defined at least in part bythe first surface complexity. For example, selection of the order ofrigid mandrel elements 130, 132, 134 along the length of articulatingstringer mandrel 112 is coordinated with the surface complexity ofcontoured surface 120 at each portion of articulating stringer mandrel112 and composite stringer 102. Portions of contoured surface 120 with ahigh surface complexity (e.g., first portion 122) require a shorterlength (e.g., first length 146) to ensure that gap 139 does not exceedthe gap threshold.

Second rigid mandrel element 132 defines a second cavity 150, anaperture 152, a second cross section perimeter 154, and a second bottomsurface 155. Each longitudinal end of second rigid mandrel element 132includes a second cavity 150 to receive a mandrel ball 138. In theexample provided, second cavity 150 is defined by a spherical surfacethat opposes each mandrel ball 138. The spherical surface accommodatesrotation and longitudinal misalignment of adjacent rigid mandrelelements 130, 132, 134 so that articulating stringer mandrel 112 mayconform to contoured surface 120. In the example provided, second cavity150 has a shape that is substantially identical to a shape of firstcavity 140.

Aperture 152 extends through a longitudinal length of second rigidmandrel element 132 between second cavities 150 on each side of secondrigid mandrel element 132. Aperture 152 accommodates cable 136 tofacilitate assembly of articulating stringer mandrel 112.

Second cross section perimeter 154 defines a shape of composite stringer102, which conforms to second cross section perimeter 154 duringautoclave curing. The shape of second cross section perimeter 154 mayvary in different embodiments based on the particular implementation anddesired stringer shape. In some embodiments, the shape of second crosssection perimeter 154 may vary along the length of second rigid mandrelelement 132. In the example provided, second cross section perimeter 154is substantially identical in shape to first cross section perimeter, islarger than first cross section perimeter 144, and has a uniform shapethrough a length of second rigid mandrel element 132. In someembodiments, second cross section perimeter 154 has a size and shapethat is similar to those of first cross section perimeter 144.

Second bottom surface 155 opposes second portion 124 of contouredsurface 120. Second rigid mandrel element 132 has a second length 156along the longitudinal direction of articulating stringer mandrel 112and composite stringer 102. Second length 156 is defined at least inpart by the second surface complexity and is larger than first length146. Although second length 156 is larger than first length 146, thesurface complexity at second portion 124 is less than the surfacecomplexity at first portion 122. Accordingly, gap 139 remains below thegap threshold.

In the example provided, third rigid mandrel element 134 smoothlytransitions between first rigid mandrel element 130 and second rigidmandrel element 134. Third rigid mandrel element has a firstlongitudinal end 160, a second longitudinal end 162, and a third crosssection perimeter 164. Third cross section perimeter 164 varies throughthe length of third rigid mandrel element 134 and is substantiallyidentical to first cross section perimeter 144 at first longitudinal end160 and is substantially identical to second cross section perimeter 154at second longitudinal end 162.

Cable 136 may be any cable capable of applying tension to articulatingstringer mandrel 112 and of withstanding the temperatures and pressuresof the autoclave process. Cable 136 extends through aperture 142,aperture 152, and mandrel balls 138. In the example provided, cable 136is a steel cable.

Mandrel balls 138 each define a pivot surface 170 and an aperture 172.Each mandrel ball 138 is disposed between each adjacent rigid mandrelelement 130, 132, 134 at least partially within cavities 140, 150. Inthe example provided, mandrel balls 138 are substantially enclosed byadjacent rigid mandrel elements 130, 132, 134 within cavities 140, 150.Pivot surface 170 opposes the surface of cavities 140, 150 and has ashape that is substantially the same as the surface of cavities 140,150. First rigid mandrel element 130 and second rigid mandrel element132 may pivot with respect to each other while maintaining alignmentwith each other using mandrel balls 138. Aperture 172 receives cable136.

Referring now to FIG. 5, a method 200 of manufacturing a compositestringer of an aircraft is provided in accordance with some embodiments.The described process steps, procedures, and materials are to beconsidered only as exemplary embodiments designed to illustrate to oneof ordinary skill in the art methods for practicing the invention; theinvention is not limited to these exemplary embodiments. Various stepsin the manufacture of reinforced composite structures are well known andso, in the interest of brevity, some conventional steps will only bementioned briefly herein or will be omitted entirely without providingthe well-known process details.

A layup tool with a contoured surface is provided in operation 202. Forexample, layup tool 110 may be provided. A stringer mandrel is providedin operation 204, where the stringer mandrel has first and secondmandrel elements with different lengths. For example, articulatingstringer mandrel 112 may be provided. Operation 206 lays skin pliesovertop the layup tool. For example, skin plies of composite skin 104may be laid overtop layup tool 110.

In operation 208, first and second mandrel elements are positioned onthe skin plies based on the different lengths, a contour complexity ofthe contoured surface, and a gap tolerance threshold between adjacentmandrel elements. For example, the order of rigid mandrel elements 130,132 along a length of articulating stringer mandrel 112 may be selectedbased on first length 146, second length 156, the surface complexity atfirst portion 122, the surface complexity at second portion 124, and thepredetermined gap threshold. Specifically, the order is selected so thatshorter mandrel elements are placed at more complex portions ofcontoured surface 120 to ensure that gap 139 does not exceed thepredetermined gap threshold.

In some embodiments, mandrel elements having different cross sectionperimeter areas are utilized. In such embodiments, mandrel elementshaving larger cross section perimeter areas are placed closer to the endof the composite stringer through which the stringer mandrel will beremoved in operation 220 described below.

The stringer mandrel is tightened with a cable in operation 210. Forexample, articulating stringer mandrel 112 may be tightened with cable136 in tension between two end mandrels of the stringer mandrel.Stringer plies are laid overtop the stringer mandrel in operation 212.For example, stringer plies of composite stringer 102 may be laidovertop articulating stringer mandrel 112 in operation 212.

The skin plies and the stringer plies are cured in operation 214. Forexample, a vacuum bag arrangement may be arranged over the stacked pliesand mandrel to apply a vacuum. Heat and pressure are applied to co-curethe skin plies and the stringer plies to form composite stringer 102affixed to composite skin 104, as will be appreciated by those withordinary skill in the art. Well known processes and conditions forapplying heat and pressure to uncured fiber reinforced materials (e.g.,an autoclave in combination with the vacuum bag arrangement) may be usedto co-cure composite stringer 102 and composite skin 104. In someembodiments, composite skin 104 and composite stringer 102 may be curedseparately.

The cable is loosened in operation 216. For example, tension in cable136 may be relieved in operation 216. The stringer mandrel is removedfrom the stringer in operation 220. For example, articulating stringermandrel 112 may be pulled out from between composite stringer 102 andcomposite skin 104 in operation 220. In the example provided, themetallic material of articulating stringer mandrel 112 has a coefficientof thermal expansion that is greater than the coefficient of thermalexpansion for the composite material. Accordingly, rigid mandrelelements 130, 132, 134 will shrink more than composite stringer 102 whencooling down from curing temperatures. By shrinking more than thecomposite stringer 102, rigid mandrel elements 130, 132, 134 create aclearance that facilitates removal of articulating stringer mandrel 112.In some embodiments, cable 136 may be pulled to remove articulatingstringer mandrel 112.

While at least one exemplary embodiment has been presented in theforegoing detailed description of the invention, it should beappreciated that a vast number of variations exist. It should also beappreciated that the exemplary embodiment or exemplary embodiments areonly examples, and are not intended to limit the scope, applicability,or configuration of the invention in any way. Rather, the foregoingdetailed description will provide those skilled in the art with aconvenient road map for implementing an exemplary embodiment of theinvention. It being understood that various changes may be made in thefunction and arrangement of elements described in an exemplaryembodiment without departing from the scope of the invention as setforth in the appended claims.

What is claimed is:
 1. A method of manufacturing a composite stringer ofan aircraft, the method comprising: laying skin plies on a layup toolthat includes a contoured surface having a first portion with a firstsurface complexity and a second portion with a second surfacecomplexity; placing an articulating stringer mandrel on the skin plies,wherein placing the articulating stringer mandrel further includes:positioning a first rigid mandrel element on the skin plies so that afirst bottom surface of the first rigid mandrel element opposes thefirst portion, wherein the first rigid mandrel element defines a firstaperture and a first stringer surface to which the composite stringerconforms, the first rigid mandrel element having a first length along alongitudinal direction of the composite stringer; and positioning asecond rigid mandrel element on the skin plies adjacent to the firstrigid mandrel element so that a second bottom surface of the secondrigid mandrel element opposes the second portion, the second rigidmandrel element defining a second aperture and a second stringer surfaceto which the composite stringer conforms, the second rigid mandrelelement having a second length along the longitudinal direction, whereinthe positioning the first rigid mandrel element is based at least inpart on the first length and the first surface complexity andpositioning the second rigid mandrel element is based at least in parton the second length and the second surface complexity so that a gapbetween the first rigid mandrel element and the second rigid mandrelelement does not exceed a predetermined gap threshold; and tightening acable that extends through the first aperture and the second aperture;laying stringer plies overtop the articulating stringer mandrel; andcuring the skin plies and the stringer plies.
 2. The method of claim 1,wherein placing the articulating stringer mandrel further includespositioning a mandrel ball between the first rigid mandrel element andthe second rigid mandrel element, the mandrel ball defining a pivotsurface on which the first rigid mandrel element and the second rigidmandrel element may pivot with respect to each other.
 3. The method ofclaim 2, wherein placing the mandrel ball further includes substantiallyenclosing the mandrel ball within a first cavity defined by the firstrigid mandrel element and a second cavity defined by the second rigidmandrel element.
 4. The method of claim 1, wherein positioning the firstrigid mandrel and positioning the second rigid mandrel are further basedon composite process characteristics that impact stepping in thecomposite stringer.
 5. The method of claim 1, wherein positioning thefirst rigid mandrel element further includes positioning a plurality ofthe first rigid mandrel element at portions of the layup tool wheresurface complexity exceeds a complexity threshold, and whereinpositioning the second rigid mandrel element further includespositioning a plurality of the second rigid mandrel element at portionsof the layup tool where surface complexity is less than the complexitythreshold, and wherein the first length is less than the second length.6. The method of claim 5, wherein the plurality of the first rigidmandrel elements and the plurality of the second rigid mandrel elementshave substantially identical cross section perimeters.